Electrostatic earth-ground potential measurement

Abstract: A motor assembly (A) cyclically moves a sample (34) longitudinally through the longitudinally aligned central bores (16, 18) of a pair of hollow electrodes (12, 14) . One of the electrodes (14) is connected with a Faraday shield (10) which electrically shields the electrodes and the sample and the other electrode (12) is electrically insulated from the Faraday shield. Preferably, the Faraday shield structure further serves as a hermetic seal for sealing the hollow electrodes against adverse effects of the ambient atmosphere. An electrical instrument (C) is connected across the two hollow electrodes to measure the induced charge transfer or potential difference as the sample cyclically moves back and forth between the hollow bores of the two electrodes.

Description: Background The present specification relates to the art of electrical potential determination. It finds particular application in determining true or absolute potential and measuring other electrostatic potentials on an absolute scale. It is to be appreciated that the present proposals are applicable to monitoring potential and change in potential relative to a standard or absolute potential reference. The potential of earth-ground or other objects, such as aircraft, fluid or grain containers, chemical reaction tanks, hospital patients, and the like may be measured. Historically, voltage measuring devices required two different electrostatic potentials in order to produce a measurement. It has been considered a common convenience and practical convention for engineers and scientists to use the Earth as a common reference point when measuring single point sources of potential. Earth-ground is arbitrarily assigned a zero potential. A convenient common reference for voltage measurements is provided by a metal rod driven into the ground, a connection to a metallic water pipe, a connection to a power line common ground wire, or the like. There are inherent inaccuracies resultant from measuring potential with reference to earth-ground. The potential of the Earth changes with time and location. Telluric currents flow along the Earth on both land and water. Telluric currents result from storms, time-varying electromagnetic induction, and other natural and man-made causes. Thus, the potential of earth-ground varies with location on the surface of the Earth and with time. Further, solar wind and energetic charged particles, often associated with sunspots, periodically flow over and immerse the Earth. Exposure of the Earth to a varying flux of charged particles can also be expected to alter the absolute potential of the Earth. Within the atmosphere, the Earth is more negative than the surrounding atmosphere, which surrounding atmosphere in turn has an altitude-dependent potential difference relative to the Earth. That is, the potential difference across the atmosphere of the Earth is about 400,000 volts. The potential difference tends to vary most quickly adjacent the Earth with a voltage variation on the order of 75 to 250 volts per meter. Pollution, humidity, snow, rain, and other atmospheric events cause variations in this potential difference. Electronic equipment in an aircraft is commonly grounded to the metal frame of the airplane and other vehicles. However, aircraft with time assume a potential more closely related to the potential of the surrounding atmosphere than to the potential of the Earth below. The potential differences between aircraft tend to manifest themselves in operations such as mid-air refueling, in which two aircraft come into physical, hence electrical contact. There are other, apparently much greater voltage or potential differences within thunderstorm clouds. From investigations of lightning and thunderstorms, it appears that some clouds can be hundreds of millions of volts different from other clouds or the ground. By arbitrarily assigning a zero potential value to earth-ground, the possibility that the Earth can have a finite and changing potential is ignored. Like birds on a power line, measurements relative to earth-ground are completely unaware of large changes in Earth potential. With a standard reference, changes in the Earth potential with time and/or location, can be measured. With a reference potential, changes in Earth potential could be measured analogous to temperatures on the Celsius scale with "zero" at a readily measurable event, i.e. the freezing point of water. In time perhaps this electrical standard can be extrapolated to an absolute zero potential in the sense of absolute zero on the Kelvin temperature scale. There are a variety of (periodic) "full moon" type phenomena which may be related to changes in the supposedly zero Earth potential. These effects include the build-up of boiler scale, the rate of polymerization of colloidal systems, and others which tend to vary with the 28 day cycle of the moon. Solar winds also vary on a 28 day cycle, mimicking the cycle of the moon. Other phenomenon such as bleeding during surgery, migraine onset, and neurological disorders follow similar cyclic patterns. Monitoring changes in the Earth potential may also alleviate recalibration and drift problems in electronic equipment, spontaneous explosions of potentially explosive materials, gremlins in the electronics of aircraft, submarines, space vehicles during launch, and the like. Other effects of Earth potential change may include blood clotting times, negative air ion benefits, crop and market cycles, reproduction rates of bacteria, and the like. Lorrain and Corson have calculated that an Earth surface charge of 8.16X10 13 coulombs could generate the Earth's magnetic field. If one multiplies the charge density for a 0.9% sodium chloride solution(observed using our techniques herein) by a 1.664X10 24 grams estimate of the total quantity of water in the hydrosphere, one obtains an electrostatic charge of 6.3X10 13 coulombs, suggesting that the hydrosphere may well contain sufficient static charge to produce the Earth's magnetic field. My earlier U.S. Patent No. 4,839,581 issued June 13, 1989 describes apparatus and techniques for measuring electrical potential. Although the apparatus described in the aforementioned patent was successful, it did have drawbacks, particularly in the area of long term drift when potential measurements were made repeatedly or intermittently over extended durations on the order of months and years. Another drawback with these prior apparatus is that their output readings were temperature dependent. The present application describes an apparatus and technique which enables an accurate determination of dielectric temperature coefficients which will provide a built-in temperature regulation. Einstein's 1924 non-neutrality of matter hypothesis has been tested in the past, generally with equipment based on the Faraday "ice bucket" experiment. In the Faraday ice bucket experiment, an insulated pewter ice-pail was connected by a wire to a delicate gold-leaf electrometer. A round brass ball was hung from and insulated by a dry silk thread. The ice bucket and electrometer were perfectly discharged and the brass ball was charged when held some distance from the ice bucket. The brass ball was introduced into the volume of the ice bucket. When the brass ball had a positive charge, the electrometer diverged positively; when the brass ball was taken away, the electrometer collapsed perfectly. As the brass ball entered the ice bucket, the divergence of the electrometer increased until the brass ball was about 3 inches below the top edge of the ice pail. Thereafter, the divergence of the electrometer remained quite steady and unchanged for any greater depression into the ice pail. This was taken as showing that after this point, the inductive action of the brass ball is entirely exerted on the interior of the ice pail and not in any degree directly upon external objects. When the brass ball touched the bottom of the ice bucket, all of its charge was communicated to the ice pail; there is no longer any inductive action between the brass ball and the ice pail. Upon being withdrawn and examined, the brass ball was found to have been perfectly discharged. Others have used Faraday cups to measure static charge. Keithley Instruments "Low Level Measurements" describes a test fixture for evaluating material for antistatic properties. A pair of conductive cups are mounted one inside the other separated by an insulator. A Model 617 electrometer is connected across the cups and set in the coulombs mode. An IC is placed in the tube and allowed to slide the length of the tube and fall into the Faraday cup. The amount of charge built up is registered by the electrometer. The test is typically repeated with the same IC and same length tubes of different materials. In this manner, different materials can be evaluated for antistatic protection. The general aim herein is to propose new and useful apparatus and measurement techniques, preferably overcoming one or more of the above-referenced problems or others. In accordance with one aspect of the present invention, an apparatus for measuring electrical potential is provided as defined in claim 1. Claims 2-6 relate to preferred embodiments of the apparatus according to claim 1. In accordance with another aspect of the present invention, a method of measuring electrical potential is provided as defined in claim 7. Still further features and effects of our invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of embodiments. Brief Description of the Drawings The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating embodiments and are not to be construed as limiting the invention. FIGURE 1 is a front plan view of a potential measuring apparatus; FIGURE 2 is a side sectional view illustrating interior components of the Faraday cage component of FIGURE 1; FIGURE 3 is a sectional view through section 3-3 of FIGURE 2; FIGURE 4 is a sectional view through section 4-4 of FIGURE 2; FIGURE 5 is a sectional view through section 5-5 of FIGURE 2; FIGURE 6 is a sectional view through section 6-6 of FIGURE 2; FIGURE 7 is an exemplary plot of net charge density (nanocoulombs/29cc) versus time (hours) for glycerol; FIGURE 8 is an exemplary plot of dielectric net charge at ground potential versus dielectric coefficient, particularly susceptibility, for various dielectric materials. With reference to FIGURE 1, a reversing electric motor and motor controller assembly A functions as a means for cyclically moving samples within a Faraday cage assembly B with an electrostatic measuring means C , such as a Keithley Model 617 Electrometer. With reference to FIGURES 2-6, the Faraday cage assembly B includes a Faraday cage 10 which provides shielding from electrical fields and gradients in the surrounding atmosphere. A pair of Faraday cup or hollow electrodes 12, 14 with one end open are mounted symmetrically within the Faraday shield. In the preferred embodiment, the Faraday cups or hollow electrodes surround circularly cylindrical inner bores 16, 18. The Faraday cups are supported with their central cylindrical bores 16, 18 in alignment. At least one of the Faraday cups 12 is supported on an insulator 20. A sample 30 is mounted for reciprocal movement between the cylindrical bores of the two hollow electrodes 12, 14. More specifically to the preferred embodiment, the sample 30 includes a plastic or other electrically insulating vessel 32 which contains a dielectric material 34 . In the preferred embodiment, the dielectric material is FC-77, a type of FLUORINERT™ perfluorinated liquid manufactured by 3M Corporation. The FC-77 FLUORINERT™ material is advantageous because it is chemically inert and stable. The FC-77 FLUORINERT™ material protects the surface of the grounding electrode within the sample bottle to prevent polarization or contamination. The thermal and electron conductivities of the FC-77 FLUORINERT™ material are sufficient to allow ground potential and temperature changes to achieve charge density equilibrium within reasonable periods of time. Other suitable materials include saline solution, methanol, glycerol, water, and the like. Because glycerol readily absorbs water, the ratio of water and glycerol in a mixture can be adjusted to create a liquid with a susceptibility of anywhere between 40 and 80. A nonconductive closure 36 seals the vessel against evaporation. To assure electrical communication between the dielectric material 34 and the Faraday cage 10, a sharp point 38 preferably extends through the closure 36 into the sample material 34. A conductive slide rod 40 is mounted for sliding movement along a guide surface 42 of a top conductive cap or plug 44 . Rather than using the electrode, slide rod, and plug to define a conductive path from the material 34 to the Faraday cage, a wire can extend through a hollow bore in the slide rod. Stop 46 and the closure 36 limit sliding movement of the support rod, hence the sample, such that the sample is movable reciprocally within the hollow electrodes without physically touching the insulated electrode 12. The motor and control assembly A includes a spool or bobbin 50 from which a string 52 extends over a pulley 54 and is connected with the slide rod 40 . A spring 56 provides resiliency to provide the motor assembly leeway between the stops reaching the limit position and the motor assembly reversing. Preferably, the controller continuously operates the motor in the reversing mode such that the sample moves cyclically back and forth between the interior wells of hollow electrodes 12 and 14 . The electrometer C is connected with the two hollow electrodes. More specifically to the preferred embodiment, a coaxial connector 60 has a central connection connected by an electrical conductor 62 with the insulated hollow electrode 12 . An outer conductor of the coaxial electrical socket is connected with the Faraday cage 10 and the hollow electrode 14. More specifically to the preferred embodiment, the hollow electrode 14 is connected by a connector 64 with the threaded top element or plug 42 of the Faraday cage. The plug 42 is connected with the outer Faraday cage cylinder 10 which, in turn, is connected with a second electrically conductive end fitting 66 of the Faraday cage assembly. The coaxial fitting 60 is connected with the lower fitting 66 to provide electrical communication between the outer coaxial socket and the Faraday cage, hence the electrode 14 . Further to the preferred embodiment, the Faraday cage 10 is a complete cylinder into which the end plugs 44 and 66 are threadedly received in a fluid tight seal. In this manner, the Faraday cage is also hermetically sealed. The hermetic seal prevents atmosphere within the Faraday cage from changing humidity, prevents insects or contaminants from entering the Faraday cage or hollow electrodes, and the like. A fitting 70 is provided for selectively controlling the atmosphere within the Faraday cage. The check valve fitting 70 may be connected with sources of known, dry gas to fill the Faraday cage with a known gaseous atmosphere. Optionally, a water jacket 72 is provided around the Faraday cage. The water jacket has ports which are interconnectable with a pump for circulating water or other fluid which is maintained at a controlled temperature through the water jacket. In this manner, the instrument can be maintained at a preselected, constant temperature. The resistance of air or other gas between the electrode and the Faraday shield is measured as well as the capacitance of the Faraday cage assembly B to assure that the charge measured is due entirely to the sample and not to off-set voltage or capacitance effects. With reference to FIGURE 7, the net charge of a recently "activated" or put into service sample material 34 is observed to approach a steady state value asymptotically. Instead of decaying to zero, each of the samples becomes stable at a characteristic non-zero charge value, regardless whether it requires hours, days, or months to achieve this equilibrium charge level. If samples are removed from the Faraday cage assembly and charged to a higher negative or positive charge level, with repeated cyclings they each return to the same characteristic charge value. With reference to FIGURE 8, the characteristic charge value of each sample is observed to be related linearly to the dielectric susceptibility (∈ r -1) or (k-1) of the sample material 34. As stated by Maxwell, the concentration of charge is equivalent to the concentration of potential. Because the apparent characteristic charge density of dielectrics is at equilibrium with the surrounding and grounded metal Faraday shield, one might assume that the shield has a potential value other than the zero which is assigned by convention to earth-ground. Stable net charge must be trapped or bound by some undetermined mechanism or else it would dissipate under coulomb forces according to Poisson's Equation. The slope illustrated in FIGURE 8 can be characterized as: Φ 0 = (Q d /volume)/(∈ 0 (∈ r -1)) where Φ 0 in volts designates an absolute reference ground potential as distinguished from the arbitrary zero convention; Q d is a net dielectric charge which is detected as a consequence of the electrostatic potential at the surface of the Earth; ∈ 0 is a reference or vacuum permittivity; and ∈ r is relative permittivity of the dielectric sample. A non-zero earth-ground potential Φ 0 polarizes molecules by increasing their electrostatic potential energy without violating the charge conservation principle. An electrostatic energy U d absorbed as a function of Φ 0 is equivalent to charge binding energy as defined by: U d = Φ 0 Q d In an example when the measured dielectric net charge Q d =0, then U d =0. Under this condition, Φ 0 is defined as zero. With the above calculated scale, the data indicates that the Earth surface absolute potential is about 500,000 volts negative. Dielectric samples measured with the apparatus indicate a temperature dependence of charge density. The charge varies in accordance with the expected sample permittivity change due to temperature. The charge density of the FC-77 FLUORINERT™ material is reduced by about 3% for each degree centigrade increase at about 25° C. When the temperature is returned to an initial value, the charge density returns to the initial value. Water, by distinction, exhibits about a 0.5% increase in charge for each degree centigrade of temperature decrease. Our concepts have been described with reference to a particular embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims: An apparatus for measuring electrical potential which includes first and second electrodes (12, 14) mounted in a Faraday cage (10); the first electrode (12) being hollow and having an opening adjacent one end, the first hollow electrode being mounted in the Faraday cage and electrically insulated therefrom; the second electrode (14) being hollow and being mounted in the Faraday cage, the second electrode having an opening in alignment with the opening of the first hollow electrode; characterized by : a dielectric sample (34) movably mounted in the Faraday cage for movement between interiors of the first and second hollow electrodes; means (A) for cyclically moving the sample between the interiors of the two electrodes ; an electrical measuring means (C) connected with the first and second electrodes for measuring one of (i) a relative potential and (ii) an induced charge flow therebetween. The apparatus as set forth in any preceding claim further characterized by the second hollow electrode (14) being electrically connected with the Faraday cage. The apparatus as set forth in any preceding claim further characterized by a means for hermetically sealing the first and second hollow electrodes and the movable sample from ambient air. The apparatus as set forth in any preceding claim further characterized by the dielectric sample including: a hollow, insulating vessel; and a dielectric fluid sealed within the dielectric vessel. The apparatus as set forth in claim 4 further characterized by a means for electrically connecting the dielectric fluid with the Faraday cage. The apparatus as set forth in any preceding claim further characterized by a temperature control means for maintaining the first and second hollow electrodes and the dielectric sample at a constant temperature. A method of measuring electrical potential between first and second hollow electrodes which are mounted in and shielded by a Faraday shield characterized by : (a) moving a dielectric sample cyclically back and forth between interiors of the hollow electrodes; (b) in each cycle, measuring one of (i) charge transfer and (ii) relative electrical potential between the two electrodes. The method as set forth in claim 7 further characterized by the moving and measuring steps being cyclically repeated until the measured potential reaches a steady state potential. The method as set forth in either of preceding claims 7 and 8 further characterized by the dielectric sample being maintained in a spaced relationship to the electrodes during the moving step (a). The method as set forth in any of preceding claims 7, 8, and 9 further characterized by the sample including at least one of: an inert fluorocarbon material, alcohol, distilled water, a water solution, plastic, wood, rubber, and ceramic. The method as set forth in any of preceding claims 7, 8, and 10 further characterized by the dielectric sample including one of a powder and a liquid sealed in a dielectric container.

Inventors: PETERSON THOMAS F JR, PETERSON, THOMAS F. JR.
Applicants: PETERSON THOMAS F, PETERSON, THOMAS F. JR.
Application year: 1994
Main classification: G01R5
IPC codes: G01R15, G01R29